A diaphragm type fuel pump adapted to operate under the influence of pulsative pressure generated in a crankcase or in a suction tube is known. Here, the structure of a conventional diaphragm type fuel pump will be described below with reference to FIG. 17. A first cover 4 including a first flexible diaphragm member 2 and an annular gasket 3 in the clamped state is arranged on one side surface of a pump casing 1, and a second cover 7 including a second flexible diaphragm member 5 and a gasket 5 in the clamped state is arranged on the other side surface of the pump casing 1. While the first flexible diaphragm member 2 and the annular gasket 3 are held between the pump casing 1 and the first cover 4 in the clamped state, and moreover, the second flexible diaphragm member 5 and the gasket 5 are held between the pump casing 1 and the second cover 7 in the clamped state, these members are immovably held by tightening a plurality of bolt members 8. Usually, the first flexible diaphragm member 2 and the second flexible diaphragm member 5 are constructed by using a rubber membrane having a base fabric involved therein. However, on occasion the first flexible diaphragm 2 and the second flexible diaphragm 5 are constructed by using a resin membrane, and in this case, the gasket 3 is additionally held between the pump casing 1 and the first flexible diaphragm member 2 in the clamped state, and moreover, the gasket 6 is additionally held between the pump casing 1 and the second flexible diaphragm member 5 in the clamped state (consequently, four gaskets in total are arranged in the fuel pump in the clamped state).
A pulsation chamber 9 is formed between the first flexible diaphragm member 2 and the first cover 4, and moreover, a pump actuating chamber 10 is formed between the pump casing 1 and the first flexible diaphragm member 2. A certain intensity of pulsation pressure generated in an engine is introduced into the pulsation chamber 9 via an introduction passage 11. Further, a fuel suction chamber 12 and a fuel discharge chamber 3 are formed between the pump casing 1 and the second flexible diaphragm member 5, and moreover, an air chamber 14, corresponding to the fuel suction chamber 12 and the fuel discharge chamber 13, is formed between the second flexible member 5 and the second cover 7. With such construction, fuel is introduced into the fuel suction chamber 12 via a fuel inflow hole 15, and fuel is caused to flow out of the fuel pump via a fuel discharge hole 16.
The pump actuating chamber 10 and the fuel suction chamber 12 are communicated with each other via a fuel passage 18 having a suction valve 17 disposed therein, while the pump actuating chamber 10 and the fuel discharge chamber 13 are communicated with each other via a fuel passage 20 having a discharge valve 19 disposed therein. The suction valve 17 serving to open the fuel passage 18 is attached to a grommet 21, and additionally, this grommet 21 is attached to the pump casing 1 in such a manner as to enable it to move relative to the pump casing 1. In addition, the discharge valve 19 serving to open the fuel passage 20 is attached to a grommet 22, and this grommet 22 is attached to the pump casing 1 in such a manner as to enable it to move relative to the pump casing 1. A coil spring 23 for biasing the first flexible diaphragm member 2 in a direction expand pulsation chamber 9 is received in the pulsation chamber 9. In dependence on the nature of the pulsation pressure introduced into the pulsation chamber 9 from the crankcase, there arises an occasion that this coil spring 23 is used, and alternately, there arises an occasion that the coil spring 23 is not used.
With respect to the conventional diaphragm type fuel pump shown in FIG. 17, die cast products obtained by using aluminum or a similar metallic material by practicing a die casting process are generally used for the pump casing 1 and the first cover 4. When there arises a malfunction that a phenomenon of vapor locking appears as fuel (especially, gasoline) receives the heat generated in the engine, there occurs an occasion that a resin material having excellent thermal insulation is used for the pump casing 1 and the first cover 4. In this case, since there arises a malfunction when creep deformation occurs on the pump casing 1 and the first cover 4 as a plurality of bolt members 8 are tightened when a thermal plastic material is used, a thermosetting resin is used for the pump casing 1 and the first cover 4. However, the thermosetting resin has poor productivity. In fact, a thermosetting resin exhibiting low creep deformation is available but it is difficult to use this material on the economically acceptable basis for the reason that it is expensive.
Another problem inherent to the conventional diaphragm fuel pump consists in the fact that the annular gasket 3 and the gasket 6 adapted to be held together with the first flexible diaphragm member 2 and the second flexible diaphragm member 5 in the clamped state are expensive. In addition, since the first flexible diaphragm member 2 and a single or two annular gaskets are clamped between the pump casing 1 and the second cover 4, the second flexible diaphragm member 5 and a single or two gaskets 6 are clamped between the pump casing 1 and the second cover 7, and finally, these members are tightened in the superimposed state, the conventional diaphragm type fuel pump is unavoidably produced at an increased cost attributable to the increased man-hours required for assembling the aforementioned members.